Field of the Invention
Embodiments of the invention relate to a transparent multilayer film containing a poly(meth)acrylimide resin layer, and a method for producing the same. In particular, embodiments of the invention relate to a transparent multilayer film containing a poly(meth)acrylimide resin layer, which is excellent in surface smoothness, transparency, appearance and punching processability, and a method for producing the same.
Description of the Related Art
In recent years, touch panels have come into wide use which are installed on image display apparatuses, such as liquid crystal displays, plasma displays and electroluminescence displays, and on which input can be made by touch with a finger, a pen or the like while a display is being looked at.
Members having glass as base materials have conventionally been used for touch panel display face plates and transparent conductive substrates, since they meet required properties such as heat resistance, dimensional stability, high transparency, high surface hardness and high rigidity. However, the glass has the disadvantages of being low in impact resistance to be liable to be broken, low in processability, difficult in handling, high in specific gravity and weighty, and difficult in meeting requirements of face curving and flexibilization of displays, and the like.
Then, materials as a substitute for glass are actively being studied; there are proposed many hard coat-laminated films in which a hard coat excellent in surface hardness and abrasion resistance is formed on the surface of a transparent resin film base material of triacetyl cellulose, polyethylene terephthalate, polycarbonate, polymethyl methacrylate or norbornene polymer or the like. However, their heat resistance and dimensional stability are insufficient.
Particularly with respect to substitutes of transparent conductive substrates, it is desired that when a transparent conductive layer is formed, the process temperature is kept high to increase the degree of crystallinity, thereby lowering the surface resistance. However, adoption of hard coat-laminated films formed from the transparent resin film base material has not been progressed for the following reasons: since the heat resistance of the transparent resin film base material is insufficient, the process temperature cannot be raised; since the heat resistance of the transparent resin film base material is insufficient, a thin film transistor cannot be further formed on a transparent conductive laminated film; and the like. It is the present situation that for transparent conductive substrates, glass is exclusively used.
Then, the inventors have studied using a poly(meth)acrylimide resin as a transparent resin film base material. The poly(meth)acrylimide resin is a thermoplastic resin which maintains characteristics, as they are, of acrylic resins of high transparency, high surface hardness and high rigidity, also has characteristics introduced thereto of polyimide resins of excellent heat resistance and dimensional stability, and is improved in a drawback of coloration from light yellow to reddish brown.
Such a conventional poly(meth)acrylimide resin is known. The usual T die extrusion method, however, cannot provide a poly(meth)acrylimide resin film excellent in surface smoothness, transparency and appearance.
The conventional art provides “an acrylic film thin in film thickness and good in die line, surface roughness and film wrinkle, and a method for producing the same.” Since the poly(meth)acrylimide resin has also characteristics of acrylic resins, the inventors have attempted the application known in the conventional art, but there cannot be obtained after all a poly(meth)acrylimide resin film excellent in surface smoothness, transparency and appearance.
When a transparent resin film base material is processed as a touch panel display face plate or a transparent conductive substrate, it is advantageous in view of the production efficiency that a punching method can be applied. It has been found, however, that the poly(meth)acrylimide resin film has a drawback to the punching processability.